I had a similar problem with the Hua Sheng 4 stroke. It was fixed by cleaning up the idle jet, and making sure the o ring on the jet was properly in place.
Remove idle screw from the carb.
The small plastic "cap" on the carb body under the idle screw can be lifted with a screwdriver, when you pull it out you will see that it is a jet.
There is a 4mm ID o-ring on the jet. make sure it is in place in the notch.
Give the jet a few sprays with WD-40 to clear it out.
If the o-ring is damaged, I have heard you can get them at Ace Hardware, 4mm ID, 7mm OD or similar.
When you reinstall the jet, make sure it is wet with WD-40 or something so the o-ring doesn't jam up as you push it down.
I will bet that this solves your problem.
My problem was solved by putting the o-ring back in the notch (it was mashed flat) and/or by spraying out the jet.